Metal powder coating apparatus

ABSTRACT

A method of coating metal pipes and strip which comprises moving a heated metal surface to be coated, bringing a plastics coating powder blend into contact with the surface and wiping the powder onto the surface. A light and flexible wiping pressure is applied and cooling air is preferably blown onto a wiping member. The surface to be coated is conveniently orientated approximately uprightly and moved downwardly. In a continuous process powder is continuously supplied to a wiping-on region in measured amount. An apparatus is disclosed for coating metal pipes and strip which comprises a flexible wiping blade which applies a wiping edge against a surface to be coated and forms a wedge shaped trough for powder between the blade and the surface. A conveyor to supply powder to the trough between surface and blade is provided and preferably an adjustable spring acts on the wiping blade to permit adjusting the wiping pressure preliminarily. A machine to move the surface to be coated is necessary and air cooling apparatus is a preferred accessory.

United States Patent 91 Brooks METAL POWDER COATING APPARATUS [76] Inventor: Donald Herbert Brooks, P.O. Box

816, Vanderbijlpark, South Africa [22] Filed: May 23, 1972 [21] Appl. No.: 255,989

[52] US. Cl 118/59, 118/101, 118/409,

' 118/413 [51] Int. Cl B44d l/OO, B44d 1/094 [58] Field of Search l18/DIG. l0, DlG 11, 407, 118/69, 408, 202, 409,59, 107, 101, 413;

[56] References Cited UNITED STATES PATENTS 1,648,475 11/1927 Darwin ll8/D1G. 11 2,334,294 l1/1943 Stevens... 118/105 3,197,324 7/1965 Brooks 118/407 X 3,389,009 6/1968 McNultyet a1 117/21 X 3,450,571 6/1969 Zenczak 117/21 X Primary Examiner-. lohn P. McIntosh Attorney, Agent, or Firm-Omri M. Behr; Peter .1. 1 Gaylor Dec. 31, 1974 [5 7] 1 ABSTRACT Amethod of coating metal pipes and strip which comprises moving a heated metal surface to be coated, bringing a plastics coating powder blend into contact with the surface and wiping the powder onto the surface. A light and flexible wiping pressure is applied and cooling air is preferably blown onto a wiping member. The surface to be coated is conveniently orientated approximately uprightly and moved downwardly. In a continuous process powder is continuously supplied to a wiping-on region in measured amount.

An apparatus is disclosed for coating metal pipes and strip which comprises a flexible wiping blade which applies a wiping edge against a surface to be coated and forms a wedge shaped trough for powder between the blade and the surface. A conveyor to supply powder to the trough between surface and blade is provided and preferably an adjustable springacts on the wiping' blade to permit adjusting the wiping pressure preliminarily. A machine to move the surface to be coated is necessary and air cooling apparatus is a preferred accessory.

3 Claims, 7 Drawing Figures PATENTEI] EH13] I974 FIGS FIG. 6

1 METAL POWDER COATING APPARATUS BACKGROUND OF THE INVENTION This invention pertains to the art of coating metal using plastics coating powder blends. The invention relates particularly, though not exclusively, to coating metal pipes internally and externally and sheet metal strip on one or both sides. In the practice of this art the surface to be coated is pretreated, (including an approved primer), and brought up to coating temperature (e.g. approx. 500 F) as rapidly as possible without oxidising or adversely affecting the primer. Powder is then applied to the surface and fuses to form the coating.

When powder is applied by the fluidised bed method, including the case of the static aerate, the homogeneous nature of the powder bed is changed immediately a large surface or intricate work-piece is immersed in the powder bed, causing uneven powder deposition.

Electrostatic spraying only givesgood results in cases not subject to screening effects and where thin coatings are required. When coating a longpipe a travelling spray head is needed'which makes an homogeneous application of the whole coating at one uniform temperature unrealizable. Post heating must be avoided in order to achieve a dense coating product.

The method of rapidly injecting a statically aerated powder into a vertical pipe was found to give good results only with short lengths of pipes (at the outside up to feet, whereas commercial lengths range'between 30 and 60 feet.)

Other systems, for example the inclined rotating pipe system with introduction of powder at the upper end, generally result in poor and uneven coatings as conditions during the coating cycle are inconsistent.

An object of this invention is to provide a new system which permits at least comparable and mostly superior results to those mentioned above and avoids the disadvantages referred to.

Another object is to apply the powder to the total area to be coated in as short a time as possible to permit achieving as uniform as possible residual heating over the total coating. The corollary is to avoid post heating to provide a dense coating, i.e., with a minimum of air inclusions. Another object is to achieve further consolidation of the coating by the action of the method and apparatus of this invention.

SUMMARY OF THE INVENTION An apparatus for coating metal in accordance with this invention comprises at least the following integers:

A flexible acting wiper blade means adapted to locate BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic end sectional elevation of apparatus in accordance with this invention in an embodiment adapted for coating the outside surface of a pipe,

FIG. 2 is a'partial front elevation of the apparatus shown in FIG. 1,

FIG. 3 is a schematic end sectional elevation of apparatus in accordance with this invention in an embodiment adapted for coating the inside surface of a pipe,

FIG. 4 is a schematic end sectional elevation of apparatus in accordance with this invention in an embodiment adapted for coating both surfaces of strip metal continuously.

FIG. 4a shows an arrangement for coating both sides of a strip in horizontal stretches,

FIG. 5 is an enlarged partial end sectional elevation of a coating region in apparatus in accordance with this invention showing the coating mechanism, and

FIG. 6 is a perspective view of a coating apparatus schematically with a stationary surface and a moving wiper blade and powder applicator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

FIGS. 1 and 2 schematically illustrate a coating apparatus for coating the outside surface 10 of a pipe 1. The

. pipe 1 is mounted at each of its two ends on a chuck 2 (of which only 1 appears in FIG. 1) having jaws 3 adjustable individually by screws 4. The jaws 3 grip against the inside surface 16 of the pipe 1 and the outside surface of the pipe 1 a flexible wiper blade 9 is arranged to exert a light, flexible pressure along its wiping edge 90, extending along the full length of the pipe 1 to be coated. Blade 9 is fixed along its anterior edge to a shaft '10 pivoted between plates 6 and biassed by spring 11. A thumbscrew or wheel 12 enables one to adjust the tension of spring 11 and so vary the pressure of the edge 9a against the pipe surface. An air nozzle 12 is shown in FIGS. 1 and 2 and is repeated at suitably spaced intervals along a long pipe to effect cooling of the blade 9. Above the blade 9 a trough 13 is pivotally mounted by means of shaft 14 between end plates 6, and its function is to dump a supply of coating powder onto the tube surface la near the blade 9 when coating begins.

The method of coating used with this apparatus is roughly as follows:

i. the pipe 1 is mounted as shown, pretreated before and/or after mounting, including primer.

ii. it is heated while blade 9 is folded away from the pipe; this preheating is done as uniformly as possible, for which reason it can be done with the pipe rotating.

It can be continued for one-half hour and the pipe I brought to the vicinity of 500 F. This temperature is determined by the fusion temperature of the coating powder blend being used, for example polyvinylchloride. This temperature can be varied according to requirements of the coating powder blend employed in order to obtain an acceptable fusion and blend into an impervious coating in an acceptable time, without post heating. Post heating should be avoided because this tends to trap air voids of minute size and so militates against obtaining a dense coating. Hence the factor of absence of post heating must be taken into account in determining the preheat temperature. The blade 9 is 3 folded away from the'pipe during preheating because it is essential that this blade be kept cool, i.e., well below the powder fusion temperature so that powder does not stickto it during coating. The edge 9a in particular must be kept sufficiently cool, and cooling air can be directed onto blade 9 during preheating.

iii. A measured supply of coating powder blend is put into the trough 13. The measure influences the coating thickness which will be obtained, and the amount of powder'per unit run of the trough obviously is'dependent on the pipe diameter.

iv. The pipe 1 is rotated in the direction of arrow 8 at a suitable speed determined empirically for each case. While an example of a speed is 40 feet per minute peripheral speed at the coating surface, the speed is rather flexible, not critical. Experimental work suggests that a wide range of speeds is possible, viz. to 400 feet per minute. Optimum speed in any given application can be a compromise between coating quality and production speed and other practical factors.

v. The blade 9 is then moved into contact with the pipe surface 1a and immediately powder is dumped into the wedge shaped trough formed between the blade 9 and surface la. Cooling of blade 9 is maintained. Pressure of the blade against the surface la is put at a value which is just sufficient to prevent escape of loose, unfused powder through the gap between edge9a and surface la, being as light as possible. An example of the order of magnitude of the pressure at the edge 9a one pound per foot run can be disclosed. Basically, however, this value must be determined empirically: if pressure. is too high blade edge 9a will Scrape against the powder coating which has fused to the pipe surface 1a, while if too low blade edge 90 will come away from the fused powder coat on surface la and allow loss of unfused powder. It must be realised that blade edge 9a exerts no influence on the thickness or uniformity of the coating formed, it is merely a keeper for the powder. The blade can flex and allow for undulation and other dimensional variations in the surface la of the pipe in its sealing action. This is fundamentally different from an earlier coating means and method in which a sheet is passed through a gap between two rigid and accurate rolls. The dimension of the gap determines the coating thickness, a plastic dam of coating material being in the nip between roll and sheet, in this old art. The blade 9 in the .present apparatus can be pressed against pipe surface la merely be pressure of the cooling air streams directed against blade 9. It should also be realised that the powder does not adhere to the blade 9 in the first place due to cooling and that just at edge 9a the coating can be wiped to encourage a dense coating. The blade 9 can be thin flexible steel or it can be a flexible plastics, for example polytetrafluoro ethylene. Sticking of powder to the tip 9a is avoided since' the powder contacted'by the tip 90 is not yet fused fully, just fusion tacked, complete fusion occurring further on. With small diameter pipes the powder can be dumped quickly, while with larger I diameter pipes powder can be poured into the wedge whole coated surface and so a uniform film quality.

7 EXAMPLE:

a. coating is a P.V.C. blend of following physical characteristics:

i. screen analysis U.S. std. mesh 7? I00 22 MO 39 ii. average particle size is microns. This powder is an example of what is called free flowing.

iii. fusion rate, determined by accumulating about a 157 mil coating thickness on l/8 inch stainless steel panels preheated 30 minutes at 525 F, being the time to flow out the accumulation to a smooth coating, is 25+ seconds. Y b. the blade is adjusted to a pressure of its edge against a pipe of llb. per foot run.

c. the pipe is rotated for a peripheral speed of 8 inches per second, and heated to a temperature of 500 F, with the blade removed.

a. the blade is moved against the tube and powder is dumped. As soon as all the powder is fused onto the pipe surface the blade is moved away from the powder surface. Rotation is continued until the powder coating has fused out into a smooth continuous film.

e. The pipe is quenched by water spray and removed from the chucks for inspection and stock piling.

As shown in FIG. 3 the method and apparatus may be adapted for internal pipe coating. The pipe 15 is in this case mounted between two chucks 16 (of which only one is seen) gripping onto the external surface of the pipe 15a. The blade 17 is pivotally mounted by shaft 18 inside the pipe with trough 19, both between nonrotating plates 20 (of which one is seen) arranged as shown. A spring 21 again provides tension for blade 20 and trou'gli 19 is pivotally mounted for dumping powder. The method of use of this apparatus is in all respects analogous to the method described with reference to FIGS. 1 and 2.

As shown in FIG. 4 apparatus in accordance with this invention adapted for coating strip comprises an arrangement of three rollers 22, 23 and 24. The sole purpose of these rollers is to obtain a stretch 25 of upright, downwardly moving strip 26 to permit coating its surface 26a which at other points is underneath. If a stretch existsalready in a production or treating line which is orientated as in stretch 25 no such provision of rollers need be specially made. The point is that no back up rollers are necessary in the apparatus in accordance with this invention for the coating operation. A blade 27 is, as an example, in this case not spring loaded by a separate coil spring, but is of thin high tensile steel which exerts a flexible, light pressure against the sheet 26. A blade 28 similarly serves along the horizontal stretch 29. FIG. 4a shows an alternative orientain accordance with this invention can thus be carried out on an upright surface moving downwards or on an upper horizontal surface. The wedge shaped troughs formed between each of the blades 27 and 28 and the strip 26 are continuously fed with coating powder, in this example by conveyor belts 33 and 34 respectively. A scraper plate 35 scrapes coating powder off belt 33 to fall into the troughlike space between blade 27 and belt 26. This plate 35 is arranged to provide a uniform delivery of powder across the width of strip 26, or substantially so. Belt 34 is arranged to simply supply powder off its end, as shown. Preheating of the'str ip 26 is schematically indicated by heater 36 and cooling of blades 27 and 28 is indicated by cooling air stream nozzles 37 and 38 respectively. Arrows show the directions of rotation and movement. The process is essentially continuous at a steady state once startedup and operating parameters have settled down. An example of the strip coating operation is the following:

EXAMPLE:

a. The strip is 24 gauge and is moved at 30 feet per minute. b. Heating capacity is adjusted to preheat the strip at this speed to 500 F (after a suitable pretreatment earlier in the line).

c. Coating powder blend is continuously supplied to the troughlike spaces formed by the wiping blades at a rate adjusted to provide a wedge shaped bed or pocket of powder whose dimension in the direction of movement of the sheet is 6 inches to 8 inches.

These parameters result in a coating being applied of the order of thickness'of mils, that is lO'thousandths of an inch.

Increasing the dimension in the direction parallel to movement of the sheet will increase the coating thickness. Variation of the strip speed and heating temperature will also influence the coating produced, as will the nature of powder blend. These factors can be reasonably determined within the skill of the strip coating art by analogy with the old apparatuses and methods.

The mechanism of coating can be further discussed with reference to FIG. 5. A metal thickness 57 is a pipe wall or a strip section. A wiper blade 58 is shown in part. The metal 57 moves in the direction of arrow 39 and a bed or pocket of coating powder40 has a free surface 41, being bounded by blade 58 and metal 57 so that the powder body is wedge shaped. In the vicinity of the blade edge 58a the wedge shape does exert an action which assists to press the powder particles into fusing contact with the heated metal 57. The powder particles begin fusing to the metal 57 from the free surface 41, and build up the thickness of the fused-on particles progressively till the edge 58a. Hence the length 42 is a parameter influencing the thickness of the produced coating, amongst others. The blade 58 presses with a pressure less than that which would scrape the forming coating but large enough to prevent escape past edge 58a of coating powder particles not fused onto the plate. Thus the blade edge 580 or its pressure has no influence as such on the thickness of the coating produced, in strong contrast to coating processes passing a sheet between two closely toleranced rollers. On

the other hand the wedge shape does exert a compacting action giving a denser coating which is absent in processes which pass a pipe or strip surface through a fluidised bed. Also the wedge shaped trough provides a facility to closely meter the quantity of powder to coat a given surface area which is absent in aerated bed methods. In the pipe coating application one can measure an exact amount of powder for each pipe. In strip coating one can deliver at a measured rate which related to the speed of the strip positively determines the amount of powder per unit area, the dimension 42 finding its own equilibrium. All these factors permit a uniform coating to be applied with simple and cheap apparatus of low tolerance manufacture.

Different powders can of course be applied to different surfaces, and multiple coat powder applications, it will'readily be perceived, can be achieved with apparatus in accordance with this invention.

Powder contamination is at a minimum with the method of this invention. A free flowing powder, not prone to lump forming, is required for the exercise of this invention.

In strip coating, analogous experimental results suggest that 24 gauge strip, for example, can be P.V.C.

coated at as much as 400 feet per minute providing a sufficient heat capacity is available for preheating to 600 F. A 4 inch powder bed in this case can provide a 2 mil (two thousandths of an inch) coating,

In addition to coating strip and pipe forms the invention is applicable to coating other surfaces over which a wiping blade can be moved, e.g flat plates and nonflat surfaces for which the blade edge can be accommodated, e.g. shallow corrugated surfaces, and others.

As shown in FIG. 6 a metal surface 43 to be coated can be stationarily mounted. A flexible blade 44 is resiliently mounted on a bin 45 which has wheels 46 adapted to permit move blade 44 and bin 45 together along a track 47 (arrow 48). Coating powder pours out a slit at 49 onto the surface 43 and is wiped along the surface 43 by blade 44. Cooling air nozzle 50 is indicated, for cooling blade 44.

What I claim is:

1. Apparatus for coating a metal article which comprises a flexible actingwiper blade, means adapted to locate a metal surface of said article to be coated adjacent the blade, so that the leading edge of said blade may press lightly against the surface, heating means for the metal surface of said article located adjacent thereto, means adapted to cause a relative wiping movement between the blade and the surface, means adapted to put a thermoplastic flowable powder coating blend onto the surface, the blade being at an acute angle with the surface to be coated in that direction to form a wedge shaped trough for the powder, and means for cooling the blade, the metal surface and the coating, said cooling means located proximate to the locus of connection between the blade, the powder, and the metal surface.

2. Apparatus as claimed in claim I, in which the blade is mounted with means adapted to permit adjusting the pressure of the blade against the surface.

3. Apparatus for coating a surface of metal pipes and strip with a thermoplastic flowable powder coating blend which comprises a flexible wiping blade stationarily mounted adapted to permit adjusting wiping pressure, means adapted to move the surface past the wiping blade, so that the leading edge of the latter may wipe across the surface located at an acute angle with the surface, the direction of the acute angle vertex being in the direction of movement, means adapted to heat the surface located adjacent thereto, and means face of the wedge shaped space between blade and sur- 8 v face and means for cooling the blade, the metal surface and the coating, said cooling means located proximate to the locus of connection between the blade, the powv adapted to put a quantity of the powder onto the sur- 5 and the metal Surface 

1. Apparatus for coating a metal article which comprises a flexible acting wiper blade, means adapted to locate a metal surface of said article to be coated adjacent the blade, so that the leading edge of said blade may press lightly against the surface, heating means for the metal surface of said article located adjacent thereto, means adapted to cause a relative wiping movement between the blade and the surface, means adapted to put a thermoplastic flowable powder coating blend onto the surface, the blade being at an acute angle with the surface to be coated in that direction to form a wedge shaped trough for the powder, and means for cooling the blade, the metal surface and the coating, said cooling means located proximate to the locus of connection between the blade, the powder, and the metal surface.
 2. Apparatus as claimed in claim 1, in which the blade is mounted with means adapted to permit adjusting the pressure of the blade against the surface.
 3. Apparatus for coating a surface of metal pipes and strip with a thermoplastic flowable powder coating blend which comprises a flexible wiping blade stationarily mounted adapted to permit adjusting wiping pressure, means adapted to move the surface past the wiping blade, so that the leading edge of the latter may wipe across the surface located at an acute angle with the surface, the direction of the acute angle vertex being in the direction of movement, means adapted to heat the surface located adjacent thereto, and means adapted to put a quantity of the powder onto the surface of the wedge shaped space between blade and surface and means for cooling the blade, the metal surface and the coating, said cooling means located proximate to the locus of connection between the blade, the powder, and the metal surface. 